We already handle n << (uint64(c)&63).
This change also handles n << (uint8(c)&63)
where the SSA compiler promotes the counter to 32 bits.

Fixes #19681

Change-Id: I9327d64a994286aa0dbf76eb995578880be6923a
Reviewed-on: https://go-review.googlesource.com/38550
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>